An ideology "at odds with traditional British values" has taken hold at the schools inspectorate Ofsted, a group of leading educationalists and Muslim leaders have warned.

Led by Sir Tim Brighouse, a former chief education officer in Birmingham, the 20 experts – unhappy at the way Ofsted has conducted inspections into schools allegedly infiltrated by conservative Muslims – say in a letter to the Guardian that it is at risk of compromising political independence by producing "tarnished reports".

Their intervention comes days before Ofsted publishes results of an inspection of 21 schools ordered by education secretary, Michael Gove, after claims conservative Muslims were trying to infiltrate the governing bodies of Birmingham schools in a plot dubbed Operation Trojan Horse.

On Tuesday, further evidence also emerged of abrupt shifts in Ofsted's inspection results, with a leaked inspection report showing that a second secondary school in the city that had been previously rated as good or outstanding in November 2013 is expected to be downgraded to inadequate when its new report is published next week.

Describing the mass inspection as "a landmark in the history of education in these islands", Brighouse and the other signatories argue: "First-hand accounts of the Ofsted inspections that have emerged are disturbing. They suggest that inspectors were poorly prepared and had an agenda that calls into question Ofsted's claim to be objective and professional in its appraisal of standards in schools serving predominantly Muslim pupils.

"It is beyond belief that schools which were judged less than a year ago to be outstanding are now widely reported as 'inadequate', despite having the same curriculum, the same students, the same leadership team and the same governing body. This is uncharted territory, with Ofsted being guided by an ideology at odds with the traditional British values which schools are meant to espouse, particularly fairness, justice and respect for others."

An Ofsted spokesman dismissed the authors' concerns: "We will be publishing our findings early next week. However, these claims are baseless. Ofsted's chief inspector, Sir Michael Wilshaw, had personal oversight of these inspections, which were carried out in very challenging circumstances, and he is completely satisfied that his inspectors conducted themselves with great professionalism and integrity throughout the process."

Brighouse was chief education officer for Birmingham from 1993 to 2002, and later became the schools commissioner for London. In both posts he oversaw rapid increases in pupil achievement. The other signatories include Robin Richardson, the former Runnymede Trust director and chief inspector of schools in Brent, Tom Wylie, former head of the National Youth Agency and a previous assistant director of inspection for Ofsted, and Salma Yaqoob, former Birmingham city council member and leader of the Respect party.

Richardson said: "This affair is going to do a great deal of harm to Ofsted's reputation, for it will be seen that they're not as impartial and independent as they like to claim, and that they do not understand the underlying issues which the media hysteria about the Trojan Horse affair has revealed.

"More importantly, I am anxious about the damage that the hysteria has done to children in Birmingham and their parents, and to their education and sense of belonging and security in British society, and their long-term interests."

The criticism comes as details of several school inspection reports continue to leak out, after Ofsted delayed their publication until later next week, citing the complexity of the task.

The delay has seen copies of the reports begin to circulate, with some schools even sending copies of results to parents in an effort to allay fears.

One secondary school, Saltley School in Bordesley Green, is to be downgraded to inadequate by Ofsted inspectors and placed in special measures – according to a copy of the inspection report seen by the Guardian – only six months after a visit resulted in a good rating in 2013.

A comparison of the two reports shows how the climate and Ofsted's priorities have changed in the space of a year. While the school's 2013 report states "all safeguarding procedures are rigorous and meet requirements. Detailed records are maintained and kept up to date", the 2014 version states: "Weaknesses in policy and procedure for safeguarding means that neither senior leaders nor governors can be sure whether students are safe."

The 2014 report mainly concentrates on the breakdown in relationships between the school's governors and staff, although it does mention the subjects that triggered the investigation: "Governors have not adopted policy and procedure to allow them to check carefully that students are safe. They … do not see any need to engage with external agencies to make sure students are safe from and aware of the risks of radicalisation and extremism."

Last week the Guardian revealed that at another Birmingham secondary, Park View, Ofsted initially issued a mildly critical inspection. But 10 days later the inspectors returned to the school and subsequently issued a far more damning report that downgraded it from outstanding to inadequate.

Other Birmingham schools investigated as part of the Trojan Horse affair have received glowing reports from Ofsted - based at least in part on their apparant efforts in public and private to combat Islamic extremism.

Inspectors have rated Ninestiles school's leadership as outstanding: "Both the executive principal and the member of the board of trustees have made public statements speaking out against extremism and, over time, have reported their concerns to the local authority, elected members and prominent politicians."

In its inspection Small Heath school was praised by Ofsted for "regular debates on current affairs, helping students to value people's different opinions as they learn about life in modern Britain", and rated as outstanding for leadership.

The inspection report for Washwood Heath Academy in Stechford, praises the school and its executive head, Bev Mabey: "The executive headteacher, and key members of the board of governors, have provided a strong lead on issues of religious extremism," the inspectors wrote.

"The executive headteacher has worked with local community groups for many years and is well respected for the contribution that she brings … Police officers believe that the academy makes a significant contribution to racial and cultural harmony within the local area."

Wilshaw confirmed that the full array of 21 reports would be published on 10 June, in a response to the Labour MP Liam Byrne, who accused Ofsted and the DfE of treating parents in his Birmingham constituency with contempt.

"This investigation is now at risk of descending into a complete shambles. The school inspections were completed weeks ago, leaks to the media have multiplied, and quite frankly parents have got a right to know the truth," said Byrne, who represents Birmingham Hodge Hill.

"Hundreds of parents now have offers for school places next year. Children are about to start their final term before exams. Yet the uncertainty goes on."